Currently, in wireless transmitters and the like, high-power amplifiers (HPAs) that amplify radio frequency (RF) signals are used. An HPA might be driven in a nonlinear region, in which a relationship between an input level and an output level is nonlinear, in order to increase the amplification efficiency. U.S. Pat. No. 7,769,103 is an example of related art.
The nonlinearity of the HPA generates, in an output signal, a parasitic component having a bandwidth that is, for example, P times (P is a natural number equal to or larger than 2) as large as that of an input signal. For example, the third-order nonlinearity of the HPA generates, in the output signal, a parasitic component having a bandwidth three times as large as that of the input signal. In addition, the fifth-order nonlinearity of the HPA generates, in the output signal, a parasitic component having a bandwidth five times as large as that of the input signal. Thus, the nonlinearity of the HPA generates nonlinear distortion (inter-modulation distortion) such as third-order distortion (IM3) and fifth-order distortion (IM5) inside and outside the band of an RF signal.
Pre-distortion (PD) is a technique in which nonlinear distortion is reduced by linearizing the HPA. In the pre-distortion, nonlinear distortion is compensated by providing the reverse characteristics of the nonlinearity of the HPA for the signal input to the HPA.
In addition, adaptive pre-distortion (APD) is known in which nonlinear distortion is compensated while the output signal of the HPA is being fed back. The adaptive pre-distortion may be, for example, adaptive digital pre-distortion in which pre-compensation of nonlinear distortion is realized by digital processing.
In the adaptive digital pre-distortion, a feedback path that feeds back the output signal of the HPA is provided. Along the feedback path, for example, an analog-to-digital converter (ADC) that converts the output signal of the HPA into a digital signal is provided. As the ADC, for example, a high-speed ADC is used to convert a feedback signal into a digital signal.
However, in the above-described related art, because a high-speed ADC has low bit resolution, a capability to compensate nonlinear distortion in the pre-distortion might be low.